In a typical cellular network, also referred to as a wireless communication system, a User Equipment (UE), communicates via a Radio Access Network (RAN) to one or more Core Networks (CNs).
A user equipment is a device by which a subscriber may access services offered by an operators network and services outside the operator's network to which the operator's radio access network and core network provide access, e.g. access to the Internet. The user equipment may be any device, mobile or stationary, enabled to communicate over a radio channel in the communications network, for instance but not limited to e.g. mobile phone, smart phone, sensors, meters, vehicles, household appliances, medical appliances, media players, cameras, or any type of consumer electronic, for instance but not limited to television, radio, lighting arrangements, tablet computer, laptop or Personal Computer (PC). The user equipment may be portable, pocket storable, hand held, computer comprised, or vehicle mounted mobile devices, enabled to communicate voice and/or data, via the radio access network, with another entity, such as another user equipment or a server.
User equipments are enabled to communicate wirelessly with the communications network. The communication may be performed e.g. between two user equipments, between a user equipment and a regular telephone and/or between the user equipment and a server via the radio access network and possibly one or more core networks, and possibly the Internet.
The radio access network covers a geographical area which may be divided into cell areas, with each cell area being served by a base station, e.g. a Radio Base Station (RBS), which in some radio access networks is also called eNB, NodeB, B node or base station. A cell is a geographical area where radio coverage is provided by the base station at a base station site. The base stations communicate over the air interface with the user equipments within range of the base stations. In the following, the term eNB is used when referring to the base station.
Multimedia Broadcast and Multicast Services (MBMS) is a broadcasting service offered via cellular networks. The MBMS is a point-to-multipoint service in which data is transmitted from a single source entity to multiple recipients. The MBMS service may be used for file download and for streaming type of services, e.g. “Mobile TV”.
Enhanced MBMS (eMBMS) is an enhanced version of MBMS and it is used to denominate MBMS service in Evolved Packet Systems (EPS) including E-UTRAN (LTE) and UTRAN access. E-UTRAN is short for Evolved UMTS Terrestrial Radio Access Network, UMTS is short for Universal Mobile Telecommunications System, LTE is short for Long Term Evolution and UTRAN is short for Universal Terrestrial Radio Access Network. eMBMS was included in the Third Generation Partnership Project (3GPP) release 9 specifications. eMBMS is related to broadcasting of content to multiple users equipments simultaneously, utilizing LTE networks. eMBMS may for example be particularly useful during live events, such as music concerts or sports events, where millions of consumers are simultaneously viewing the same content, and where eMBMS may be used to broadcast complementary content, like different camera angels for instance, to LTE user equipments. eMBMS enables operators to make better use of their available spectrum and free up network capacity. Thus, the operators may maximize efficiency when offering services such as live TV, video on demand, podcasts etc.
One concept in eMBMS is the MBSFN transmission. MBSFN is an MBMS Single Frequency Network. A MBSFN area comprises multiple cells in which transmission of identical waveforms is performed at the same time. A property of MBSFN transmission is that all participating cells transmit exactly the same content in a synchronized manner so it appears as one transmission to the user equipment. This gives the possibility for user equipments to combine MBMS transmissions from multiple cells. Transmitting the same data to multiple user equipments allows network resources to be shared. Mechanisms are therefore provided to ensure synchronization of the MBMS content—i.e. to ensure that all participating eNBs include the same MBMS control information and data in the corresponding time-synchronized subframe.
To achieve the MBSFN transmission, the following synchronizations are needed:                Network synchronization        MBMS User Data flow synchronization        MBMS control plane synchronization (also called MCCH Update Signaling synchronization)        
MCCH is short for Multicast Control Channel and is a point-to-multipoint downlink channel used for transmitting MBMS control information from the eNB to the user equipment. This channel is only used by user equipments that receive MBMS.
The eMBMS is realized in the 3GPP specifications by the addition of a number of new capabilities to existing functional entities of the 3GPP architecture and by addition of a new functional entity, a Multi-cell/multicast Coordination Entity (MCE).
According to 3GPP, there are two eMBMS deployment alternatives:                Alternative 1: Standalone MCE, see FIG. 1.        Alternative 2: Distributed MCE, see FIG. 2.        
Alternative 1 with the standalone MCE will now be described with reference to FIG. 1. FIG. 1 is an illustration of the eMBMS logical architecture of a communications network 100 with a standalone MCE. The communications network 100 comprises a LTE core network 100a and a LTE radio access network 100b. 
The Broadcast Multicast Service Center (BM-SC) 101 is an entity which controls MBMS sessions and corresponding MBMS bearers.
In this figure, the MCE 103 is a logical standalone entity. The functions of the MCE 103 are the admission control and the allocation of radio resources used by all eNBs 105 in the MBSFN area. The standalone MCE 103 is involved in MBMS Session Control Signaling. The standalone MCE 103 decides when eNBs 105 perform MCCH update signaling to user equipments (not shown). Accordingly, the MCCH update signaling synchronization may be achieved. Only two eNBs 105 are shown in FIG. 1 for the sake of simplicity, but the skilled person will understand that more than two eNBs 105 may also be comprised in the communications network 100.
The Mobility Management Entity (MME) 107 is a control node in the communications network 100.
MBMS GateWay (MBMS GW) 110, is an entity that is present between the BM-SC 101 and eNBs 105 whose functions is the sending/broadcasting of MBMS packets to each eNB 105 transmitting the service. The MBMS GW 110 performs MBMS Session Control Signaling (Session start/stop) towards the E-UTRAN via the MME 107.
The content provider 113 provides eMBMS services to the communications network 100.
The M3 115 is the interface between the MCE 103 and the MME 107, and is a control plane interface as indicated by the dotted line. M1 117 is the interface between the MBMS GW 110 and the eNBs 105, and is a user plane interface as indicated by the continuous line. M2 120 is a control plane interface between the MCE 103 and the eNBs 105. IP multicast 123 is used for point-to-multipoint delivery of user packets from the MBMS GW 110 to the eNBs 105.
Alternative 2 with the distributed MCE will now be described with reference to FIG. 2. FIG. 2 is an illustration of the eMBMS logical architecture of a communications network 200 with a distributed MCE. The communications network 200 comprises a LTE core network 200a and a LTE radio access network 200b. 
The Broadcast Multicast Service Center (BM-SC) 201 is an entity which controls MBMS sessions and corresponding MBMS bearers.
In this FIG. 2, the MCE is a distributed entity which is a part of another network element, i.e. the eNB. In FIG. 2, the combined MCE and eNB is referred to as an eNB/MCE 203.
The Mobility Management Entity (MME) 207 is a control node in the communications network 200.
MBMS GW 210 is an entity that is present between the BM-SC 201 and eNB/MCE 203 whose functions is the sending/broadcasting of MBMS packets to each eNB/MCE 203 transmitting the service. The MBMS GW 210 performs MBMS Session Control Signaling (Session start/stop) towards the E-UTRAN via the MME 207.
The content provider 213 provides eMBMS services to the communications network 200.
The M3 215 is the interface between the MCE part of the eNB/MCE 203 and the MME 207, and is a control plane interface as indicated by the dotted line. Thus, the architecture in FIG. 2 may be referred to as being a M3 based architecture. M1 217 is the interface between the MBMS GW 210 and the eNB part of the eNB/MCE 203 and is a user plane interface as indicated by the continuous line. IP multicast 223 is used for point-to-multipoint delivery of user packets.
For the MBMS control information, whenever the standalone MCE 103, see FIG. 1, updates the control information it indicates the modification period from which the updated control information applies by means of a parameter called MCCH update time, as illustrated in FIG. 3. The MCCH update time 301 and the MCCH modification period 305 is indicated in FIG. 3. This concept is used in the standalone MCE architecture to synchronize control plane signaling: MCCH update signaling for all eNBs 105 by sending the same MCCH update time 301 to all eNBs 105. Hence, the synchronization of the MCCH update signaling for all eNBs 105 may be achieved. The range of the MCCH update time 301 is 255.
The 3GPP standard supports control plane synchronization for the distributed MCE architecture by including the parameters “Time of MBMS Data Transfer” and “Time of MBMS Data Stop” in the MBMS session start request and the MBMS session stop request messages respectively. The “Time of MBMS Data Transfer”/“Time of MBMS Data Stop” is an absolute timestamp which indicates the absolute time of the actual start or stop of the MBMS data transfer. Accordingly, all eNBs/MCE 205 will transfer/stop user data at the same time.
In the case of distributed MCEs where the MCE is co-allocated in the eNB 203, achieving the MCCH update signaling synchronization is not completely supported by the current standard.